1. Field of the Invention
The present invention relates to the circuitry for the diversity unit in an FM receiver intended for a telephone system, the circuitry comprising at least two channels for receiving signals possibly differing from one another in both amplitude and frequency, each channel comprising a mixer and a band-pass filter for forming an intermediate-frequency signal, and the circuitry also includes at least one phase-lock loop which includes a phase comparator, as well as summation means for the summation, for the purpose of further detection, of the channel signals which have been equalized in phase and frequency.
2. Description of the Relevant Art
In cellular telephone systems, repeaters are usually required to have diversity reception capability for two or more channels. In the NMT 900 network to be opened, the repeater using diversity reception must be capable of functioning also in the event that in one of the receiving channels no signal is received even for a long period, or in the event that the signal is below the preset threshold level. This is the situation when, for example, one of the receiver antennas or the front section of the receiver has been damaged, or when the antenna is entirely missing.
Owing to the very high selectivity required of a receiver and to the real tolerances of the intermediate-frequency filters, the intermediate-frequency signals of the channels of the diversity receiver must be capable of being tuned in each channel separately precisely for the band of the filters in the channel in question. Thus the intermediate-frequency signals are not of the same frequency in the different branches.
Among the diversity reception principles presented in the literature, maximal gain predetection combining implemented for high or intermediate frequency is the one with the best efficiency values. In its technical implementation it is also by far the most complicated and most expensive. The next best is equal gain predetection combining, which in its technical implementation and price is more economical than the former.
One method of implementing the combining is to use a phase-lock for the phasing of the signals.
Equal gain predetection combining of signals of different intermediate frequencies can be implemented by using in both diversity channels separate phase-locks which are locked either to each other or to a fixed reference. The phase and frequency of the signals to be summated are equalized by means of mixers and voltage-controlled oscillators before the summation and detection. Either a fixed-frequency reference or the summated signal itself is used as the other input signal for the phase comparators.
FIG. 1 depicts such a known circuitry using a fixed-frequency reference, and in the circuitry of FIG. 2 the summated signal itself serves as the input signal. In the figures, the RFA and RFB signals may be, for example, the signals received from two receiver antennas, their frequencies, phases and amplitudes possibly differing from one another.
In the circuitries, there are the following blocks: 1a and 1b are mixers, 2a and 2b are band-pass filters, 3a, 3b and 4a, 4b are amplifiers, 5a, 5b are phase comparators, 6a, 6b are loop filters, 7a, 7b are voltage-controlled oscillators, 8 indicates a summation circuit, 9 an amplifier, 10 a detector, 11 a loudspeaker, 12 a fixed-frequency oscillator, and 13 a phase shifting circuit (0.degree./90.degree.).
The signal of each channel A, B is delivered, after filtering and amplification, to the phase comparator 5a, 5b, the output of which is dependent on the frequency difference and the phase difference between the input signals. The output signal of the phase comparator is forwarded via the loop filter to the voltage-controlled generator 7a, 7b, the output signal of which is mixed in the mixer 1a, 1b with the input signal RFA, RFB. Owing to the phase-lock loops the signals will be of the same frequency and of the same phase before their summation in circuit 8, and their detection. In the case according to FIG. 1, the reference signal for the phase comparators is taken from the fixed-frequency crystal oscillator 12, whereas in the case according to FIG. 2 the summated signal itself is taken as the second input signal for the phase comparators after the amplifiers by means of feedback.
When a fixed crystal oscillator (FIG. 1) is used, the dimensioning of the loop filter so that the frequency modulation in the received signal will not be attenuated constitutes a problem. Because of modulation, the loop filter 6a, 6b must be be very narrow. This makes the locking speed of the loop low, in which case at high fading speeds the phase-locks will not be able sufficiently rapidly to phase the signals coming from the various channels, and the efficiency of the diversity unit will decrease.
When the summated signal itself (FIG. 2) is used as the second input signal for the phase comparators 5a, 5b, the problem is how, in the static situation, to implement with sufficient precision the phase ratio (0.degree. . . . 90.degree.) required by the phase comparators between the signals to be compared. Another problem is how the phase comparators 5a and 5b will behave when the intermediate-frequency signals are at different frequencies at the moment of start, in which case the signal in one input of the phase comparator contains intermodulation results and harmonic results of the intermediate frequencies, their mutual amplitude and frequency ratios varying as a function of the input level of the RF signal.
Thus the diversity receivers, known from the literature, using equal gain predetection combining prior to the phase-lock and detection function poorly when signals of different frequencies, containing phase or frequency modulation, are combined. Furthermore, the alternatives presented do not function optimally when one of the channels is completely devoid of a received signal or when the level of the received signal is below the set threshold level.